1. Field of the Invention
The present invention relates in general to a propulsion device for a watercraft, and in particular to a multiple-jet propulsion device.
2. Description of Related Art
Many watercraft now employ inboard-mounted jet propulsion units due to several distinct advantages over propeller type propulsion systems. For instance, no open propeller poses a hazard with a jet propulsion unit. The unit also does not detract from the watercraft's exterior appearance.
The thrust performance of a jet propulsion unit, however, is commonly limited because the impeller tends to cavitate when driven at a rotational speed above an upper limit. Cavitation reduces the efficiency of the impeller and thus the thrust performance of the jet propulsion unit.
Some prior watercraft have employed several jet propulsion units in order to fully utilize the power output by a high-horsepower engine. The large engine thus drives multiple jet propulsion units, but at a rotational speed that does not cause meaningful cavitation. That is, the engine drives each jet propulsion unit at a rotational speed below the designed upper limit and, thus, cavitation does not occur to such a degree that the efficiency of the jet propulsion unit suffers. The propulsion system thus can provide more thrust without losing efficiency.
Several prior watercraft designs, which employ multiple jet propulsion units, have located the units in a side-by-side arrangement, behind a pair of centrally disposed, juxtaposed water inlets which are located on the underside of the watercraft hull. The use of water inlets, which are arranged as close to the center line of the watercraft as possible, reduces the tendency of the jet propulsion units to draw in air when the watercraft turns. These openings, as well as at least a portion of the associated inlet ducts directly behind the openings, commonly are integrally formed with the watercraft hull.
Despite the desire to position the inlet openings closely together, prior watercraft have not done the same with the impeller housings, which receive water from the inlet ducts. One reason for this is that the impeller housings are secured to the hull by fasteners. This arrangement requires space for the fasteners, and the impeller housings cannot be as closely positioned together as are the inlet openings.
In addition, prior impeller housings have been sufficiently spaced apart to accommodate water supply and drain hoses. The water supply hose connects to impeller housing at a point downstream of the impeller and receives pressurized water through a water tap. The hose extends along the side of the impeller housing and into the hull wherein it is connected to a water jacket of the engine. Similarly, the drainage hose is connected to the impeller housing at a point upstream of the impeller and extends into the hull along side the impeller housing. The drainage hose extends into a bilge of the hull for removing water therefrom. As a result, the spacing between the impeller housings has been greater than the spacing between the inlet openings on prior watercraft in order to accommodate the hoses.
Each inlet duct thus must bend outward, away from the center line of the watercraft, in order to align with the mouth of the associated impeller housing. The resulting curvilinear path through the inlet duct increases the resistance (i.e., drag) of water flow through inlet duct, thereby decreasing the efficiency of the propulsion unit.
The pump housings are also usually affixed to an upstanding wall of the hull. The wall, however, does not provide a rigid support on which to mount the pump housings. Flexure of the wall is not uncommon. Misalignment (i.e., non-parallelism) between the jet pump units occurs which results in power loss because the thrust of the jet pump units are not oriented to optimize propulsion efficiency. For instance, a downward orientation of the jet pump units causes a portion of the produced thrust to rise the aft end of the watercraft rather that propel the watercraft forward.